ABU DHABI - Geology.

The Cretaceous: The succeeding Early Cretaceous Thamama rocks are dominated by shallow water carbonates of remarkable, widespread homogeneity. These rocks are of significant commercial importance and comprise a series of porous, clean, pellet and fossiliferous limestones with inter-bedded tight (often stylolitic lime) mudstones and packstones. They comprise, in ascending order the Habshan, Lekhwair, Kharaib and Shuaiba formations. These are better known by their informal oilfield nomenclature of Thamama Zones I to VI offshore, or Zones A to F onshore. (According to oilfield practice the zones are numbered or lettered from top downwards in order of penetration). Shuaiba differs in that it contains referral build-ups of rudists surrounded by dense basinal limestones of the Bab member.

Following Thamama Group deposition, there has been a break in sedimentation in Abu Dhabi, with the hiatus most pronounced in the south-eastern area where there was active erosion. The Thamama is succeeded by the Middle Cretaceous Wasia Group consisting, in ascending order, of the Nahr Umr, the Mauddud and the Salabikh (or Shilaif) and its partial lateral equivalent, the Mishrif. The Nahr Umr is the first major non-carbonate formation in the succession and consists of shales and minor glauconitic siltstones which pass laterally to limestones in the Oman mountain front to the east.

Further north in the Gulf, Nahr Umr is a sandstone of major importance as a reservoir, particularly in Iraq. Clean shelfal limestones of the Mauddud overlie the Nahr Umr. They are thinly developed in Abu Dhabi. Following the Mauddud, deep water subsidence once again affected the region with a north-south trending basin forming beneath Abu Dhabi. In this basin thick bituminous "oligestinal" limestones of the Shilaif formation were deposited, initially across the entire area. Later basinal conditions were restricted to the axis. Clean, shelfal and rudistic Mishrif limestones were deposited on the flanks of the basin which gradually became more constricted. In the centre of Abu Dhabi, the culmination of the basinal phase resulted in the deposition of the Tuwayil and Ruwadha members.

Middle Cretaceous deposition closed with some uplift and renewed subsidence resulting in the deposition of the Late Cretaceous Aruma Group. This group's basal member, the Laffan, is a widespread homogeneous brown-to-red open marine shale capping the Mishrif and Shilaif. In north-eastern Abu Dhabi and adjacent parts of Dubai there is evidence that the Mishrif was subject to sub-areal erosion prior to the deposition of the Laffan, and the latter shows some fresh-water characteristics. The Laffan is succeeded in turn by the Halul or Ilam limestones, the Fiqa shale and the Simsima carbonate. With rare exceptions, these are not commercially important in Abu Dhabi. Both the Fiqa and Simsima lose their characteristic identity in eastern Abu Dhabi and pass into the Gurpi formation, a unit deposited in a basin forming in the northern UAE.

The Jurassic: Around the close of the Middle Jurassic, waters deepened; and the Late Jurassic sequence begins with the deposition of deep water limestones and limy mudstones of the Diya (or Dukhan) formation which has been identified as the major source of the oils in Abu Dhabi. This unit passes laterally eastwards into more shelfal limestones.

Near the close of the Upper Jurassic, shallow water cyclic sedimentation was re-established with the deposition of the carbonates and evaporites of the Arab and succeeding Hith formations. These can be easily distinguished in western parts of Abu Dhabi. But they lose their separate identities to the east. In particular the Hith, which is a well defined anhydrite in the west, passes laterally into dolomites to the east. The end of the Jurassic was marked by a period of non-deposition and possibly local erosion.

The Upper Permian-Lower Triassic Khuff formation, where gas accumulations have been discovered in the north-western offshore areas of Abu Dhabi, remains an interesting subject of study. The formation was first studied in 1958 by Western geologists (M. Steineke, R.A Bramkamp and N. J. Sander) at the type locality of Ain Khuff near the Riyadh-Jeddah road in Saudi Arabia.

The Permian covers the upper part of the Palaeozoic era (after the Pennsylvanian), thought to have covered a period between 280 and 225 million years ago. It also covers a corresponding system of rocks. It is named after the Russian province of Perm west of the Urals mountains, where rocks of this age were first studied.

At Ain Khuff, the Khuff formation is 562 feet thick, with alternating lithology from top to bottom being as follows: (1) a 92.5-foot thick aphanitic layer of calcarenitic limestone, commonly marly and fossilifer-ous; (2) a 233-foot thick aphanitic limestone; and (3) a 110.5-foot thick layer of dolomite and shale, with angular granitic sand and fine conglomerate at the bottom. At Ain Khuff, the reported age of the formation is Late Permian and the underlying rocks are of the Pre-Cambrian basement. Wajid sandstone and Saq sandstone, with discomformable contact, and the overlying rocks are of the Lower Triassic Sudair formation.

During the Middle Permian in Abu Dhabi, the climate gradually became warmer and more arid. As a result of marine transgression, a carbonate platform was established throughout the region, on which the thick sequence of shallow-water carbonates and subordinate evaporites of the Khuff were deposited. These ranged in thickness from 2,668 to 3,026 feet. The formation was completely penetrated in a total of 17 Khuff structures in Abu Dhabi, in which ten large gas discoveries have been made in the offshore areas. Consisting mainly of slightly anhydritic dolomite and limestone, the formation is divided into Upper and Lower Khuff, which are separated by a widespread bed of 40/60-foot thick anhydrite. The latter is called Middle Anhydrite. The Upper and Lower Khuff are subdivided into reservoir units, with Upper Khuff having four units and Lower Khuff having three units. All are based on significant log picks, sedimentological support, well and outcrop data.

Khuff reservoirs vary from well to well and from field to field. The Upper Khuff is more productive than the Lower Khuff. High gas flow rates are due partly to production from fractures in zones of poor porosity development in the Lower Khuff. The absence of hydrocarbons in the onshore Khuff structures could be due to the fact that most of the existing structures were developed after the main period of hydrocarbon migration.

Source Rocks: The basinal limestones of the Diyab formation are the source of most of the oil and gas now contained in Abu Dhabi's Jurassic and Cretaceous reservoirs. The Shilaif is a high quality potential oil source rock; but in Abu Dhabi, it has not reached sufficient maturity to have generated significant amounts of oil. In areas where its depth of burial is sufficient to have reached maturity, such as eastern-most Abu Dhabi and western Dubai, it may have sourced the oils in the Mishrif at Umm Al Dalkh and the offshore Dubai fields.

In terms of volume, the Thamama is the most important of Abu Dhabi's reservoirs. The unit produces over a wider area of Abu Dhabi than any other. It forms the producing reservoirs of Bab, Bu Hasa, Asab, and Sahil in the onshore, and Zakum and Mubarraz in the offshore. Also in the offshore Thamama, oil occurs at Umm Shaif and Abu Al Bukhoosh, in addition to their older reservoirs. In the onshore, production is mainly from the younger Thamama reservoirs, while in the offshore, at Zakum, the older zones have had the longer production history. The low permeability Zones I and II were developed more recently despite their vast volumes of oil in place. Cap gas is associated with the oil in Bab, where there has been a major gas/condensate development.

The Araej, comprising upper and lower members with the intervening Uwainat group, is an important reservoir in the western and central offshore. Light oil is present in Umm Shaif. But elsewhere only non-associated wet gas occurs.

The Arab formation is also of major importance in the offshore areas, especially at Umm Shaif, subdivided into Zones A, B, C and D, with the latter the most significant.

Porosities range up to 30% and permeability can exceed 100 md.

Despite its seemingly attractive development as a reefal facies built-up over large areas of Abu Dhabi, the Mishrif has failed to live up to expectations.

Small accumulations of oil appear in the Halul and Asmari formations at Mandous, in the Halul at Al Khair - both offshore and onshore in the Simsima at Shah. In the case of Mandous, the oil is likely to have migrated from deeper source rocks along faults associated with the Mandous salt plug. The source at Al Khair and Shah may be more localised.

Pre-Khuff clastics and Khuff, especially Upper Khuff carbonates, form huge non-associated gas reservoirs in offshore Abu Dhabi, mainly beneath Umm Shaif.

A suitable hydrocarbon source may be present in the Silurian shales, which have been penetrated in offshore Abu Dhabi, in western Oman, and in the Qusaybah formation in Saudi Arabia.

Khuff gas is predominantly dry and the content of inert components, nitrogen and carbon dioxide increases towards the east, to such an extent that Zakum gas is judged non-commercial. The decrease in gas quality appears to be related to depth and temperature. The Khuff reservoirs are frequently of moderate porosity with low matrix permeability - the latter enhanced by natural fractures.

The Seals: The Khuff and pre-Khuff have sufficient intra-formational tight units to provide their own internal seals. The ultimate seals for the Khuff are the shales and evaporites of the overlying Sudair. The basinal limestones of Diyab provide top seal for the Araej, but intra-formational seals also are present. The ultimate seals for the Arab are the Hith anhydrites. Their spread may be a major controlling factor in petroleum distribution in Abu Dhabi. No "pre-Hith" oil occurs in areas east of the Hith edge. Where the Hith occurs, the amounts of Thamama oil are reduced. In these areas the occurrence of post-Hith oil can be explained by either local faulting which has breached the Hith seal, or the Thamama oil is close enough to the Hith margin to have leaked around it.

The Nahr Umr proved to be a very efficient seal for the vast amounts of Thamama oil; the tight, stylotised limestones between the producing zones also act as intra-formational seals. Where the Nahr Umr may have been breached by faulting, oil has escaped up to younger horizons; but these cases are few and far between.